Many factors in lightwave communication systems make the optical signal propagation polarization dependent. These factors include polarization-dependent loss and polarization mode dispersion in the fiber and the system components.
For example, consider polarization mode dispersion (PMD). The core of an optical fiber is not truly symmetrical, and therefore the propagation of an optical signal at one polarization, for example, vertical, will be different than the propagation of the optical signal at another polarization, for example, horizontal. With no polarization-dependent loss, for each frequency there exist a pair of orthogonal input states of polarization for which the corresponding output states of polarization are orthogonal and are independent of wavelength to the first order. These states are referred to as the principle states of polarization (PSP) in the fiber. With sufficiently narrow bandwidth, such as with external modulation of a single-frequency laser, optical signals transmitted in either of these two states are undistorted at the receiver, but have, in general, different time delays. A signal with arbitrary polarization can be expressed in terms of a sum of signals in each PSP, and thus will be received as two signals with different delays. The received signal is therefore distorted unless it is transmitted with one of the two PSPs.
If the PSPs of the fiber remained constant, then one-time corrective measures could be taken when a system is installed to avoid its adverse effects. However, the PSPs as well as the time delay changes with time. Factors that cause the PSPs and delay to change with time include changes in temperature caused by, e.g., sunlight and ocean currents. In addition, any change of position or movement of the optical fiber will cause a change of PSPs and also a change in the time delays. Thus, for a given input polarization the received signal distortion varies with time.
With large PMD, or with wider bandwidth signals, such as with direct laser modulation, or with polarization-dependent loss, the signal propagation may no longer be adequately described by the PSP model. However, the distortion of the received signal is still polarization dependent. Furthermore, since, for a given input state of polarization, the state of polarization of the optical signal in the fiber changes with time, the distortion of the received signal also changes with time, even when the distortion is due to components whose polarization-dependent properties are constant.
It is recognized that if the polarization-dependent distortion of an optical pulse signal traversing an optical fiber can be minimized, then the bit rate with which, and/or distance over which, information can be transmitted over a fiber optic transmission channel can be increased.